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Osmosis is the phenomenon of water flow through a semi permeable membrane that blocks the transport of salts or other solutes through it.

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Introduction

Osmosis Osmosis is the phenomenon of water flow through a semi permeable membrane that blocks the transport of salts or other solutes through it. Osmosis is a fundamental effect in all biological systems. It is applied to water purification and desalination, waste material treatment, and many other chemical and biochemical laboratory and industrial processes. When two water (or other solvent) volumes are separated by a semi permeable membrane, water will flow from the side of low solute concentration, to the side of high solute concentration. The flow may be stopped, or even reversed by applying external pressure on the side of higher concentration. In such a case the phenomenon is called reverse osmosis. ...read more.

Middle

= cRT where c is the molar solute concentration, R is the gas constant, and T is the absolute temperature. This formula is the same as the pressure formula of an ideal gas. Figure-1 shows connected vessels separated by a semi permeable membrane. If there is only water in the device, the level will be the same at both sides. When solute molecules are added to one side, water will start to flow into it, so that its level will go up at this side, and down at the other side. The system will stabilize when the osmotic pressure is balanced by the hydrostatic pressure generated by the difference in the water levels. ...read more.

Conclusion

This discussion of the flow mechanism usually does not appear in textbooks that deal with osmosis. The effect of the osmotic pressure on the free surface of the solution was first suggested by Hulett in 1902 but received little attention. It seems to have only few proponents since then. Osmosis is a reversible thermodynamic process. That is, the direction of water flow through the membrane can be reversed at any moment by proper control of the external pressure on the solution. Contrary to that, mixing a teaspoon full of sugar in a cup of tea is an irreversible thermodynamic process. There is no way to reverse the process at any given moment and un- mix the sugar back to the spoon. Reversibility is a most important idea of thermodynamics. Osmosis is reversible, while disolving sugar in water, essentially diffusion, is irreversible. ...read more.

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